You can use Affinity Photo to make adjustments and apply filters to your photos directly from the Photos app. By using adjustment layers, you can test out different changes and play with settings to get the result you want.
If you need to open PDF files or just want a decent image editing app that doesn't cost too much or require a subscription, there are plenty of options in the Mac App Store and elsewhere. All of these and 64-bit apps that work with macOS Catalina and beyond. See https://macmost.com/mac-photoshop-alternatives.html for links.
It seems that uploading to Instagram is something you can only really do from your phone. But by tricking the site into thinking your Mac is a phone, you can easily upload photos and viddos to Instagram in Safari on your Mac.
If you want to combine photos into a collage, you can do it using Pages. By using Page Layout mode with custom page dimensions, rulers, guides, box shapes and alignment tools, you can create a grid of photos that at any size. Then you can drag and drop pictures from the Photos app into each box. You can also use borders or even build creative designs with various shapes and sizes.
THe Photos app includes tools like albums and tags to help you organize your photo library. But if you like, you can skip that and just use built-in features that automatically group your photos by dates, locations, people or objects.
The Photos app on your Mac has a set of adjustment and editing tools. Get familiar with how these work by experimenting with them.
You can use the Mac Photos app to highlight one color in a photo and desaturate the rest. You can also do different things with different colors. A third-party app like Pixelmator Pro can be used as an external editor to saturate a color in only part of an image.
You can quickly and easily made a slideshow in the Photos app, but those can't be exported to share. By using the free iMovie app from Apple, you can build a slideshow from a set of photos in minutes and then export it as video to share. You can also customize each photo with filters, cropping, movement and text. You can add music, audio and narration to the slideshow too. The basic idea works on the iPad as well.
Sometimes you need to shrink an image in order to upload it to a web site or send to someone. You can do this easily on a computer, but it is harder to do on an iPhone or iPad. This Shortcut will give you a way to do it by sharing the photo to create a smaller file.